Mandated Export: Why?

[Alex Buckley]

This isn't a JPMS issue per se; it's the JPMS following a broader pattern.

The distinction between ACC_SYNTHETIC and ACC_MANDATED was introduced by 
the MethodParameters attribute in Java SE 8. It is extremely useful to 
be able to distinguish between "something in the class file that a 
compiler put there as an implementation artifact, to assist other class 
files generated by the same compiler" (ACC_SYNTHETIC) and "something in 
the class file that a compiler was required to put there by a JCP 
specification, because for whatever reason the thing could not be 
denoted in source code" (ACC_MANDATED). No-one should rely on the 
presence or semantics of an ACC_SYNTHETIC artifact, but everyone can 
rely on the presence and semantics of an ACC_MANDATED artifact. There's 
a long history of ACC_SYNTHETIC being set for artifacts as if the former 
description is appropriate when in fact the latter description is 
appropriate; the prime example is fields of an enum type.

In principle, every artifact in the ClassFile structure should allow its 
reason-for-being to be flagged. For modules, it's clear that the 
JLS-defined dependency on java.base should be ACC_MANDATED, while a 
dependency added by a tool (because the tool added classes to a module) 
would be ACC_SYNTHETIC. There aren't yet any exports which would be 
ACC_MANDATED, but exports added by a tool as in the previous sentence 
would legitimately be ACC_SYNTHETIC.

Alex

On 7/21/2016 4:02 PM, Paul Benedict wrote:
>  From the document that describes the binary module format for exports [1]:
>
> 0x1000 (ACC_SYNTHETIC)
> Indicates that the dependence was not explicitly or implicitly declared in
> the source of the module declaration.
>
> 0x8000 (ACC_MANDATED)
> Indicates the dependence was implicitly declared in the source of the
> module declaration.
>
> I'd like to find out why ACC_MANDATED is necessary because it seems
> ACC_SYNTHETIC is enough. What's keenly interesting is that the description
> of ACC_MANDATED is nearly the same to what the JVMS says about
> ACC_SYNTHETIC for a class: "it was generated by a compiler and does not
> appear in source code" [2].
>
> It appears something is incorrect with the description of these flags. What
> is ACC_MANDATED really for and how it is useful?
>
> [1] http://cr.openjdk.java.net/~mr/jigsaw/spec/lang-vm.html#jigsaw-2.3
> [2] https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html
>
> Cheers,
> Paul
>